The present invention relates generally to a power transmission device operable to selectively transfer torque between first and second sets of drivable wheels of a vehicle. More particularly, the present invention is directed to a power transmission device adapted for use in motor vehicle driveline applications having an actuator including an electric motor drivably coupled to a gerotor for providing pressurized fluid to a piston acting on a friction clutch.
Due to increased demand for four-wheel drive vehicles, many power transmission systems are typically being incorporated into vehicle driveline applications for transferring drive torque to the wheels. Many vehicles include a power transmission device operably installed between the primary and secondary drivelines. Such power transmission devices are typically equipped with a torque transfer mechanism for selectively transferring drive torque from the primary driveline to the secondary driveline to establish a four-wheel drive mode of operation. At least one known torque transfer mechanism includes a dog-type lock-up clutch that may be selectively engaged for rigidly coupling the secondary driveline to the primary driveline when the vehicle is operated in four-wheel drive mode. Drive torque is delivered only to the primary driveline when the lock-up clutch is released and the vehicle operates in a two-wheel drive mode.
Another type of power transmission device is operable for automatically directing drive torque to the secondary wheels without any input or action on the part of a vehicle operator. When traction is lost at the primary wheels, four-wheel drive mode is engaged. Some transfer cases are equipped with an electrically-controlled clutch actuator operable to regulate the amount of drive torque transferred to a secondary output shaft as a function of changes in vehicle operating characteristics such as vehicle speed, throttle position and steering angle. Typically in the power transfer device is a clutch positioned within the transfer case housing.
While many power transfer devices are currently used in four-wheel drive vehicles, a need exists to advance the technology and recognize the system limitations. For example, the size, weight and packaging requirements of the power transmission device may make such systems cost prohibitive in some four-wheel drive applications.
The present invention provides a power transmission device including a friction clutch operable to selectively transfer torque between an input member and an output member. An actuator is operable to provide an actuating force to the friction clutch. The actuator includes an electric motor having an output shaft drivingly coupled to a gerotor. The gerotor is operable to provide pressurized fluid to a piston acting on the friction clutch. The gerotor substantially dead-heads and the output shaft of the electric motor rotates between 0-100 rpm during actuation of the friction clutch.
In one embodiment, the power transmission device includes a controller operable to determine a magnitude of torque to be transferred. The controller controls the actuator to pressurize fluid within a closed cavity containing a piston acting on a friction clutch to generate the requested magnitude of torque. The controller is operable to vary the supply of electrical energy to the motor via pulse width modulation to vary the output of a positive displacement pump and vary the output torque of the friction clutch. The motor is operable to continuously rotate while torque is being transferred by the friction clutch.